Circuits with thin layers of transistors (T.L.T) are mainly used in the production of active matrix display screens. In this type of screen, an electronic memory formed of memory points distributed over the entire surface of the screen stores the video signal throughout the duration of the image. The electro-optical transducer (for example, a liquid crystal) is in contact with each memory point and is excited throughout the duration of an image, whereas, in systems without any electronic memory, the transducer is only excited during the duration of a line. The optical effect and the authorized multiplexing rate are accordingly much more significant.
The TLTs make it possible to embody such an electronic memory on a glass substrate. Each memory point is situated at the crossing of one line and one connection column and is constituted of two parallel transistors and a capacitor. In the case where the transducer is a liquid crystal, the reinforcements of the capacitor may be constituted by the electrodes of the liquid crystal cell itself. The memory point is thus brought back to two TLTs and one capacitor, one of the reinforcements of said capacitor being constituted by an electrode disposed on the wall of the cell which contains the TLTs, the other reinforcement being constituted by the counter-electrode disposed on the other wall of the cell.
Such a structure is represented on FIG. 1; part A is a top view of one point of the active matrix and part B is a sectional view along the line b-b of the part A.
Part A shows a conductive column 2 and a conductive line 4 constituting the addressing columns and lines of the points of the active matrix, two TLTs 6 and 7 in parallel, and one transparent electrode 8 constituting one of the reinforcements of the capacitors or elementary display points of the active matrix.
Each column 2 is provided with a cross 10 and each electrode 8 with a finger 12. The crossings of the line 4 with the column 2 and the cross 10 defining the drains D of the TLTs and the crossing of the line 4 with the finger 12 define the source S of the TLTs. The part of the line 4 situated between the column 2 and the cross 10 constitute the grid G of the TLTs.
Part B shows firstly a lower insulating and transparent wall 20 bearing the column 2, the finger 12 of the elementary display point and the cross 10; these conductive elements 2, 10 and 12 are covered by a layer of hydrogenated amorphous silicon and then an insulating grid layer 24 supporting the conductive lines 4. This unit is covered with a self-aligning layer 26 intended to be in contact with the electro-optical material 28 (liquid crystal).
In addition, the display screen includes a transparent counter-electrode 30 supported by an upper insulating transparent wall 32.
For color display, colored filters 34, usually about three (red, blue and green), need to be provided on the counter-electrode 30 and be in contact with the material 28.
Next to the active matrixes, other circuits may be embodied with TLTs and, for example, all or part of the registers to be offset.
This screen is intended to be lit up via the rear with fluorescent tubes and display is designed to be observed via the lower face, as shown on part B of FIG. 1. The grid metal, namely the one constituting the lines 4, ensures protection of the TLTs 6 against the rear light of the display screen. On the other hand, the "underside" of the TLTs is exposed to the ambient observation light. If this light is intense, a photo-current is generated in the amorphous silicon layer 24 reducing the contrast of the screen as the Ion/Ioff ratio diminishes; Ion and Ioff respectively represent the currents delivered by the transistors of one displayed point and one non-displayed point.
By using amorphous silicon of low thickness (&lt;50 nm), this photo-current is attenuated but is nevertheless adequate so as to reduce the contrast. Also, an optical mask needs to be provided under the transistors so as to use the screen with the ambient light and thus ensuring a constant image.
In order to make an optical mask absorbing the light under the TLTs, a metal is generally used which is engraved. As the optical mask is a conductor, a passivation layer is required. Such a structure is described in the article in Japan Display 1983, p. 211 by Canon.
This technique requires the use of two depositings under vacuum, which is relatively constricting, as well as a specific masking level so as to define the form and location of the optical mask under the transistors. This additional masking level strictly requires self-aligning accuracy of this mask with respect to the source and drain contact of the transistors. Accordingly, this technique is complex and delicate.
The object therefore of the invention is to overcome these drawbacks by providing an optical mask of the "underside" of the transistors with an active matrix, while reducing the number of masking levels and by ensuring a self-positioning of these optical masks with respect to the sources and drains of the transistors.